This invention relates generally to optical scanners and more particularly to an optical scanner for creating a flying spot linear trace of a beam of laser light. The invention finds particular use in the field of laser beam scanners as are used for reading information from a copy board and directly transferring the read information for the exposure of photosensitive surfaces as in the production of printing plates.
In the aforementioned cross-referenced application Ser. No. 522,103, there is shown an apparatus for producing an exposed photo plate from a copy board paste-up. A laser scanning system having a read laser beam is focused to a spot scanned across the copy board in a predetermined pattern, such as a raster-like scan, the reflection from the copy board being sensed, read and used to control the intensity of a second laser beam via a modulator. The second laser beam impinges upon and scans a photosensitive surface. The read laser beam and the write laser beam are combined and passed through deflection optics, and the two beams are subsequently separated to impinge upon and be focused at the copy board and photosensitive surface, respectively. In this way there is a resultant exposure of the photosensitive sur face in accordance with the copy. As shown in Ser. No. 522,103, the scanning optics employed utilizes a moving mirror galvanometer, with both the read and write laser beams being aligned and superimposed upon each other through suitable beam combining optics for being passed through the galvanometer simultaneously and subsequently separated by suitable beam deflection optics to the respective planes. Another optical system shown therein employs a polygonal scanning wheel having a plurality of surfaces parallel to the axis of rotation of the wheel, with the surfaces serving to scan the read and write beams through an angle, thereby creating a flying spot scan.
In the other cross-referenced application Ser. No. 695,921 there is disclosed a variation of laser read/write apparatus in which a facsimile system is enveloped. As disclosed therein, a duplication of read and write equipment at separate locations can be coordinated to form a facsimile transmission system. At the read station an optical scanner scans the input copy with the scanning spot and the reflected light produces a video read data signal, a portion of which is directed through a spatial mask to provide a transmitter video reference which gates a video read data before transmission. In the receiver, a second optical scanner of similar construction is controlled by a video write data signal. The video write data signal gates a scanning spot of exposure laser beam light on and off to expose the output photosensitive copy surface at the receiver. Additionally, the scanning light is detected through a further spatial mask to provide a receiver video reference signal utilized to form a video write signal. The spatial masks in the transmitter and receiver have a known relationship, e.g., so that the scanning of the output copy in the receiver can be spatially synchronized with the scanning of the input copy in the transmitter. As therein disclosed, each of the scanning optics includes a galvanometer-operated mirror for scanning the incident laser beam back and forth through a horizontal angle.
The foregoing instruments as disclosed in the cross-referenced application employ a field-flattening lens for causing the beam provided from the scanning device to be focused at the plane of the copy board and photosensitive surface respectively, and are known therefore as flat bed scanners. The scanning optics, however, are subject to a number of errors which degrade the performance of the system. In a polygonal drum scanning design, very close tolerances are required during the manufacturing processes so as to control facet-to-facet tilt. Any error in facet-to-facet orientation, together with bearing run-out errors and the like, contribute to produce an angular or positional error component normal to the scan line. This error has come to be known as "wobble" or vertical error. In addition, the scan efficiency of a polygonal drum scanning system is limited to about 50 percent. Accordingly, the polygonal design is expensive to produce due to the tolerances required, and the facet-to-facet error has to be removed by some suitable means, termed a "dewobbler".
In a resonant or oscillating galvanometer scanner, the mirror pivots in a sinusoidal manner, and only the center portion of the scan is linear enough to be utilized. This results in a scan efficiency of approximately 50% with a 25% deviation in exposure or scan velocity. However, it is necessary to scan back and forth in opposing directions in order to maintain this efficiency level. Such scanning requires lag compensation which is accomplished by deviating the read beam from its normal course as a function of system time delays and scan velocity. Such compensation adds to the cost and complexity of the system and in many instances is only partially effective. In addition, if multiple machines are to communicate in a facsimile system, a great deal of calibration of each machine is required to normalize the amount of lag produced in each machine. Lag errors and other errors in the facsimile process when scanning in both directions, result in left writing and right writing images that are no loner superimposed, resulting in severe image degradation for even small errors. Further, at the higher speeds particularly associated with facsimile systems, the scanner requirements exceed the capabilities of a galvanometer mirror system because of the high torque to which the mirror and its support structure are subjected.
Other existing systems utilize cylindrically curved fields but are also limited in scan efficiency. For example, in one such system using a spinner-type scanner in a cylindrical configuration, one scan is accomplished for each rotation of the scanning device. With the exposure times commonly associated with a standard printing format, extremely high rotational speeds are required, and synchronization of facsimile versions is difficult. Furthermore, such curved field systems require that the exposure surface be adaptable to a curved conformation which is often incompatible with printing plate production.
Ideally, a scanning system should provide a high scan efficiency, a scanning operation in a single direction so as to eliminate the problem of lag, and a constant scanning velocity so as to reduce the cost of the associated electronics. In addition, the system should be free of vertical error or wobble and should be entirely reflective so as to above aberration errors caused by the read and write beam frequencies being at different portions of the spectrum. Additionally, such a scanning system should be cmpatible with flat field optics so that the resulting flying spot scan can read copy and expose plates lying on plane surfaces.